Mixer's are great but it's incredibly easy to get batter or flour all over the place if you're not careful. You can always buy a commercial splash guard, but cooking blog The Sweet Adventures of Sugarbelle suggests it's much easier to just cut a hole in a Tupperware lid.

By cutting the lid of a cheap container you can create a simple splash guard for mixers of all types. Cut out a large circle in the middle and a slit on the side to slide it over the mixer. This means the next time you're baking a cake you won't have to deal with the mess of flour that exploded everywhere when you stopped paying attention for a split second. This example uses a standup mixer, but it can certainly be used for handheld mixers as well.